The invention relates to optical devices for receiving incident light and for producing resultant light containing optical images. More particularly, the invention relates to optical devices in which the optical image contains information concerning the genuineness of a document or item.
Photocopy reproduction technologies have made significant developments in the last decade. These developments have improved the quality of reproductions as well as lowered the costs of reproducing documents. As a consequence, it has become increasingly easier for those inclined to produce counterfeit documents to do so. Relatively inexpensive photocopy machines can today produce high quality color copies which may not be discernable from their originals.
As a security measure, authenticating means are now incorporated into official documents to foil would-be counterfeiters. For example, negotiable instruments, currency papers, etc., might include optical patterns which can be seen only with infrared or ultraviolet light; and identification or validation cards might include optical interference patterns for producing optical images in the visible, ultraviolet or infrared spectrums.
Producing official documents having optical interference patterns, such as holographic embossings, is a favored technique because such patterns are difficult to duplicate. Typically however, official documents with such authenticating means are expensive to produce in large quantities. A significant portion of this cost is due to the fact that embossed optical interference patterns must be protected from damage during handling, storage and use.
For example, surface embossing techniques, such as are disclosed in U.S. Pat. No. 4,250,217, generally result in an external polymeric surface embossed material. The embossment on such a device is vulnerable to dirt particles and other foreign substances which might damage the embossed relief pattern. Not only could such particles absorb some incident light (thus rendering the polymeric material less translucent), but such substances could also destroy the embossed relief pattern by eroding or abrading it.
Prior art techniques for protecting embossments include excavating an optical pattern through a non-absorbing coating. For example, U.S. Pat. No. 4,626,445 discloses surface excavation techniques as well as the excavation of an optical pattern 1) through a substantially non-absorbing coating, and 2) through a substantially non-absorbing substrate. These techniques, however, are costly and time consuming.
There is thus a conflict between the need to produce official documents which are difficult to copy yet which can be mass-produced inexpensively.
Accordingly, it is an object of the present invention to provide an optical authentication device which is relatively inexpensive to manufacture yet difficult for others to duplicate. A related objective of the present invention is the provision of means for inexpensively preserving films containing surface embossed interference patterns.